The present invention relates to an attachment member detachably attachable to an exchangeable camera lens which is adapted to be used with a camera body having through-the-lens (T.T.L.) light measuring system, and more particularly, pertains to an attachment member for transmitting information as to the maximum f-number value or minimum diaphragm aperture available for the exchangeable lens to a camera body.
In full-aperture light measurement through an exchangeable lens which is detachably mounted on a camera body, there are two types of methods of transmitting information as to the set diaphragm aperture of the lens to the camera body. The first type requires two transmitters, one for transmitting information as to the minimum f-number (full aperture value) of the lens, and the other for transmitting information as to the set f-number (diaphragm aperture value) of the lens. The second type of method necessitates only a single transmitting member which transmits information as to the step difference from the minimum f-number to the set f-number.
The present invention is directed to the latter type of information transmittal. In this type of information transmitting mechanism, it is possible to control the exposure time in accordance with scene brightness and the set f-number of the lens by taking into consideration the fully open aperture through which the scene brightness is measured, as well as to indicate, before the diaphragm is stopped down, the exposure time to be determined or executed. However, this type of mechanism is not applicable for the indication of a diaphragm aperture to be controlled automatically in accordance with scene brightness and a selected, manually set shutter speed as the single transmitting member only transmits information as to the step difference but not information as to the minimum f-number of the lens which is necessary for indication of the f-number or of diaphragm aperture value to be controlled automatically.
To make an exchangeable lens having the second type of information transmission mechanism available for camera bodies provided with a shutter-priority automatic diaphragm control, it was proposed in U.S. pat. application Ser. No. 769,832 of Kuramoto et al filed on Feb. 17, 1977, which was assigned to the same assignee as that of the present application, to provide the exchangeable lens with a second information transmitting member to transmit information as to the maximum f-number to the lens so that information as to the minimum f-number may be obtained in the camera body by combining this information with the information as to the step difference transmitted by the first transmitting member when the diaphragm aperture setting member is set to a maximum f-number or automatic aperture control position. However, this proposed mechanism requires an additional member to be incorporated into the exchangeable lens and thus conventional exchangeable lenses having only the single transmitting member cannot provide information for the indication of automatic aperture control.